A low noise amplifier and a detector are used together in order to detect a weak electrical wave in a millimeter wave field. For example, a Schottky diode is used for this detector.
FIG. 1A illustrates one example of current-voltage characteristics of a Schottky diode. The horizontal axis indicates a voltage, while the vertical axis indicates a current. The positive direction of the horizontal axis corresponds to a backward voltage. The dotted line 100 in FIG. 1A illustrates normal current-voltage characteristics. When a forward voltage exceeds an offset voltage Vos, the current abruptly rises. Thus, it is difficult to obtain sufficient detection characteristics in a range where an input voltage is equal to or lower than the offset voltage Vos.
The solid line 101 in FIG. 1 indicates current-voltage characteristics when a bias for correcting an offset Vos is applied. In this case, even if the applied forward voltage is infinitesimal, the current rises abruptly. However, a backward current Ir increases when the backward voltage is applied. Accordingly, the detection characteristics deteriorate.
FIG. 1B illustrates one example of current-voltage characteristics of an Esaki diode. The horizontal axis indicates a voltage, while the vertical axis indicates a current. The positive direction of the horizontal axis corresponds to a forward voltage. When a forward voltage is applied, electrons tunnel from a conduction band of an n-type layer to a valence band of a p-type layer. Moreover, when the forward voltage is further increased, electrons stop tunneling. This is because an energy level at a lower end of the valance band of the n-type layer is at a band gap of the p-type layer. As a result, a negative resistance appears.
Applying a backward voltage makes a current flow because electrons in the valence band of the p-type layer tunnel to the conduction band of the n-type layer. Thus, a backward current flows without exhibiting an offset voltage such as in a Schottky diode. Therefore, non-linear characteristics between the voltage and the current are obtained.
A peak current appears in a voltage range that is lower than a voltage in which a negative resistance appears under a forward bias. This peak current is observed as a backward leak current when an Esaki diode is used as a detector.
A backward diode is known that suppresses tunneling of electrons from a conduction band of an n-type layer to a valence band of a p-type layer under a forward bias. FIG. 1C illustrates one example of current-voltage characteristics of a backward diode. A current is lowered when a forward bias is applied compared with a current of an Esaki diode. Using a backward diode as a detector may better suppress a backward leak current compared with an Esaki diode.
Moreover, a backward diode that uses a p-type GaSb and an n-type InAs is known (for example, refer to Patent application publication No. 2003-518326).
FIG. 2 is an energy-band diagram of the backward diode. An AlSb layer with a thickness that allows tunneling of electrons is disposed between an n-type InAs layer and a p-type GaSb layer. Although, in actuality, energy band bending is caused near an interface between each layer, this is not illustrated in FIG. 2. A valence band of a p-type GaSb layer and a conduction band of an n-type InAs layer partially overlap at both sides of the AlSb layer.
When a positive voltage is applied to the n-type InAs layer, electrons in the valence band of the p-type GaSb layer are transported to the conduction band of the n-type InAs layer by tunneling, as indicated by the solid line arrow. An energy level of electrons at the lower end of the conduction band of the n-type InAs layer is within a band gap of the p-type GaSb layer under a state in which a given size of a positive voltage is applied to the p-type GaSb layer. Hence, the current does not flow.
The backward diode may obtain favorable detection characteristics by causing interband tunneling.
It is assumed that an infinitesimal positive voltage is applied to the p-type GaSb layer of the backward diode illustrated in FIG. 2. When an applied voltage is infinitesimal to the extent that the energy level of electrons at the lower end of the conduction band of an n-type InAs layer is in the valence band of the p-type GaSb layer, electrons in the conduction band of the n-type InAs layer are transported to the valence band of the p-type GaSb layer by tunneling as indicated by the dashed arrow in FIG. 2. Thus, sufficient detection characteristics may not be obtained in the infinitesimal voltage range.